Transparency on Linked Groups in OpenGL
Unless I missed it, this has not yet been fixed. I actually reported this as bug once and that's how I learned it's considered a "known limitation", not a bug. Viewport transparency on dupligroups just straight up isn't supported.

If you really need to see past an object in a dupligroup, the max draw type on the source object is still respected, so you can set this to wireframe or bounding box. (max draw type is located in the object > display panel too, right next to the transparency checkbox)